The Air Force adjusted the pressure inside cabins of U-2s to 15,000 feet after a study showed the aircrafts' pilots were more prone to brain lesions. (1st Lt. Victoria Porto / Air Force)

A U-2 pilot is helped into a full-pressure suit before flight. The suits are worn because U-2s fly at altitudes of up to 70,000 feet. (Senior Airman Alexander Recupero / Air Force)

The Air Force has re-pressurized the U-2 spy plane’s cabin after a study revealed U-2 pilots had brain lesions at a higher than average rate, officials said.

The re-pressurization was completed in June along with other cockpit modifications, such as reinforcing frames, bulkheads and the canopy, said Lt. Col. Tadd Sholtis, a spokesman for Air Combat Command.

Over the past decade, the higher ops tempo for the U-2 has coincided with an increase in the number of pilots suffering from neurologic decompression sickness, also known as “the bends,” Sholtis said. Between 2006 and 2010, the number of reported incidents of decompression sickness increased from 0.076 percent to 0.23 percent of missions flown.

“By late 2010, officials believed there was enough cause for concern to initiate a study,” Sholtis wrote in an email. “What we knew or suspected about the problem was briefed to decision makers in early 2011, and the study was authorized in March 2011.”

Between May 2011 and October 2012, the Air Force conducted MRIs on 105 U-2 pilots, ranging in age from 26 to 50, said Dr. Stephen McGuire, a neurologist and retired Air Force colonel who led the study. Of those pilots, 75 percent had more brain lesions than they should for their age and current health. These are the same type of lesions caused by repeated head trauma.

“We believe these lesions are a direct result of exposure to extreme altitudes,” McGuire told Air Force Times.

U-2 spy planes often fly at altitudes above 70,000 feet, requiring pilots to wear pressurized suits to survive. Inside, the cabin is pressurized to close to 30,000 feet, or roughly the elevation of Mount Everest, according to the Air Force.

Interestingly, the pilots with the most time in the U-2 were not always the ones with more brain lesions, he said. Instead, the main contributing factors to brain lesions included how much nitrogen the pilots were able to get out of their system while breathing 100 percent oxygen before missions, how long the missions were and how much time pilots had to recover from missions.

It is not known whether the U-2 pilots are at greater risk of having health problems down the road, but Air Combat Command still felt the need to take precautions, McGuire said.

“In all other neurological disease, these types of lesions are associated with cognitive impairment,” he said. “We have not seen any clinical impairment in any of our U-2 pilots, and they are all still flying. But, being conservative, the commanders rightly made the decision, ‘Let’s not push the envelope; let’s protect our pilots.’ ”

One of the changes was reducing the pressure inside U-2 cabins to 15,000 feet, McGuire said. Research has shown that the risk of developing decompression sickness becomes “basically non-existent” below 18,000 feet.

“What we’ve done is bring the pilots down below what is commonly thought to be the threshold for DCS [decompression sickness] and we think that will prevent any further occurrence of lesions,” he said.